This proposal is for a longitudinal study of 500 phencyclidine users. The initial and follow-up interviews will be scheduled one year apart. The research design includes interviews with heavy, moderate and occasional users. Snowball sampling procedures will be used to produce a sampling frame from which a quota sample, representing the three types of users, will be selected. Local contacts will be used to assist in the identification of the sample. The final sample will include equal numbers of the three types of users. The objectives of the research are to: 1) examine patterns of use and changes in use patterns, 2) describe the perceived motivation for use, drug preferences and perceived effects of phencyclidine, and 3) to determine behavioral changes in the life of the phencyclidine user.